Add functions to calculate temperature of particles.

See merge request !697

Update the small cosmo volume YAML files with the correct parameter names for the EAGLE cooling model.

Clarified expression for delta_z in the cooling routine. Added function stubs for temperature calculation.

Added derivation of comoving Euler equations to cosmology theory document.

See merge request !590

Eagle cooling

See merge request !593

Declared all the cooling tables sizes as compile-time constants. Renamed many variables. Clarified CGS nature of many variables.

Fixed typo in Zeldovich run script.

See merge request !693

Correctly interpolate the metal heating table in all cases. Add interpolation functions ignoring the x-dimension. Move the metal species as the first element in the table.